Earth's Dynamic Plates: Formation, Movement, Boundaries

Ocean Formation and Rift Valleys

Under the pressure of hot material, the lithosphere rises. The tension created at the top causes the central area to sink, forming a Central Valley (rift formation). Magma erupts from the rift, solidifying and expanding the valley. This process forms oceanic lithosphere, initially creating a narrow sea. If the process continues, it can lead to the formation of a vast ocean.

Understanding Global Plate Tectonics

A global synthesis of ocean plate tectonics helps us determine the movements of continents and oceans, and the origin of volcanic ridges, islands, and earthquakes. The lithosphere is divided into many rigid fragments called lithospheric plates, ranging between 50 and 200 km in thickness.

The edges of lithospheric plates may be:

• Ridges: A limit where new lithosphere is generated.
• Subduction Zones: A limit where lithosphere is destroyed.
• Transform Faults: A limit where lithosphere is neither created nor destroyed.

Oceanic lithosphere is continually renewed, while continental lithosphere is permanent. Lithospheric plates move over the plastic sub-lithospheric mantle. The movements of lithospheric plates are caused by thermal energy inside the Earth, aided by gravitational energy and convection currents. Throughout Earth's history, not only the position of lithospheric plates has changed, but also their shape and size.

Forces Driving Lithospheric Plate Movement

The primary forces moving lithospheric plates are:

• Thermal Energy Inside the Earth: This causes turbulent mantle convection currents, which are the main drivers of plate movement.
• Gravity (for two reasons):
  • The oceanic plate is elevated at ridges and sinks in subduction zones, causing it to slide down.
  • In the high-pressure mantle, density increases, so the subducted edge pulls the plate down due to gravitational pull.

Lithospheric Plates and Their Boundaries

The lithosphere is the Earth's outer layer, characterized by cracks, with each piece forming a plate. The border between two plates is called a plate boundary. There are three main types:

• Oceanic Ridges: A plate boundary where new lithosphere is generated. Deep Earth materials rise here, forming new crust.
• Subduction Zones: A plate boundary where one lithospheric plate is destroyed by displacing beneath another. This process generates earthquakes and heat, leading to volcanic activity.
• Transform Faults: A plate boundary where lithosphere is neither created nor destroyed. One plate moves laterally relative to the other.

Seabed Age and Sediment Thickness

The age of the seabed rocks provides crucial insights:

• Oceanic Ridges: Basalts at oceanic ridges are younger than rocks found millions of years away. The age of the oceanic crust increases as we move away from the ridge.
• Sediment Thickness: The thickness of sediments is directly related to the age of the ocean floor. As we move away from the ridge, sediment thickness increases.